Oligodendrocytes, the myelinating glia in the central nervous system (CNS), are differentiated chiefly from neuroepithelial cells of the ventral neural tube. In ventral neural tube-derived neuroepithelial cells, a helix-loop-helix transcription factor, Olig2, is activated in response to ventrally derived sonic hedgehog (SHH). Alberta J, et al., “Sonic hedgehog is required during an early phase of oligodendrocyte development in mammalian brain,” Mol. Cell. Neurosci. 18:434-41 (2001); Fu H, et al., “Dual origin of spinal oligodendrocyte progenitors and evidence for the cooperative role of Olig2 and Nkx2.2 in the control of oligodendrocyte differentiation,” Development 129:681-693 (2002); Ligon K, et al., “Olig gene function in CNS development and disease,” Glia 54:1-10 (2006); Lu Q, et al., “Sonic hedgehog-regulated oligodendrocyte lineage genes encoding bHLH proteins in the mammalian central nervous system,” Neuron 25:317-329 (2000); Rowitch D, et al., “Sonic hedgehog regulates proliferation and inhibits differentiation of CNS precursor cells,” J. Neurosci. 19:8954-8965 (1999); Zhou Q, et al., “The bHLH transcription factor Olig2 promotes oligodendrocyte differentiation in collaboration with Nkx2.2,” Neuron 31:791-807 (2001); and Zhou Q, et al., “Identification of a novel family of oligodendrocyte lineage-specific basic helix-loop-helix transcription factors,” Neuron 25:331-343 (2000). Interestingly, Olig2 is expressed in a restricted domain of the spinal cord ventricular zone that sequentially gives rise to motor neurons and then oligodendrocytes (i.e., sequential model). That is, Olig2-expressing neuroepithelial cells in the spinal cord give rise to motor neurons during a neurogenic phase. Thereafter, these Olig2-expressing progenitors down-regulate neurogenic transcription factors such as Ngn2 and Pax6, and begin to express Nkx2.2, an oligodendroglial transcription factor. Fu et al., supra; and Qi Y, et al., “Control of oligodendrocyte differentiation by the Nkx2.2 homeodomain transcription factor,” Development 128:2723-2733 (2001). Thus, this dual function of Olig2 is controlled by spatio-temporal changes in the domains of expression of several other transcription factors in relation to Olig2.
In addition to ventrally derived oligodendrocyte precursor cells (OPCs), a smaller population of OPCs are generated from the dorsal neural tube. Battiste J, et al., “Ascl1 defines sequentially generated lineage restricted neuronal and oligodendrocyte precursor cells in the spinal cord,” Development 134:285-293 (2007); Cai J, et al., “Generation of oligodendrocyte precursor cells from mouse dorsal spinal cord independent of Nkx6 regulation and SHH signaling,” Neuron 45:41-53 (2005); Fogarty M, et al., “A subset of oligodendrocytes generated from radial glia in the dorsal spinal cord,” Development 132:1951-1959 (2005); and Vallstedt A, et al., “Multiple dorsoventral origins of oligodendrocyte generation in the spinal cord and hindbrain,” Neuron 45:55-67 (2005). While dorsally derived neuroepitheilial cells appear SHH-independent, they nevertheless must express Olig2 to become OPCs. Cai et al., supra.
Although one would predict that human OPCs could be obtained via methods akin to those used in other species, such as mice, independent laboratories have been consistently unsuccessful in generating OPCs from expanded human neural stem/progenitor cells using those methods. Chandran S, et al., “Differential generation of oligodendrocytes from human and rodent embryonic spinal cord neural precursors,” Glia 47:314-324 (2004); Roy N, et al., “Identification, isolation, and promoter-defined separation of mitotic oligodendrocyte progenitor cells from the adult human subcortical white matter,” J. Neurosci. 19:9986-9995 (1999); Windrem M, et al., “Fetal and adult human oligodendrocyte progenitor cell isolates myelinate the congenitally dysmyelinated brain,” Nat. Med. 10:93-97 (2004); and Zhang S, et al., “Tracing human oligodendroglial development in vitro,” J. Neurosci. Res. 59:421-429 (2000). Human embryonic stem cells (hESCs) have been reported to differentiate to OPCs after expansion of hESC-derived neural progenitors in the presence of either FGF2 or EGF or both. Izrael M, et al., “Human oligodendrocytes derived from embryonic stem cells: effect of noggin on phenotypic differentiation in vitro and on myelination in vivo,” Mol. Cell. Neurosci. 34:310-323 (2007); Kang S, et al., “Efficient induction of oligodendrocytes from human embryonic stem cells,” Stem Cells 25:419-424 (2007); Nistor G, et al., “Human embryonic stem cells differentiate into oligodendrocytes in high purity and myelinate after spinal cord transplantation,” Glia 49:385-396 (2005). It is unknown, however, whether the growth factors specify OPCs by deregulating the dorsal-ventral patterning as in mice or simply expand spontaneously differentiated OPCs.
As in other vertebrates, human OPCs express Olig2. Jakovcevski I and Zecevic N, “Olig transcription factors are expressed in oligodendrocyte and neuronal cells in human fetal CNS,” J. Neurosci. 25:10064-10073 (2005). However, whether Olig2 transcription is necessary and how neural progenitors transform to OPCs in humans remain uninvestigated. Answers to the above questions are instrumental to an understanding of the biology of human neural development, as well as to an ability to promote remyelination in humans. As such, additional methods of generating oligodendrocytes from human embryonic stem cells (hESCs) are needed in the art.